1. Field of the Invention
The present invention relates to a positioning system, lithographic apparatus and method for manufacturing a device.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In some systems, there is a positioning system used to position a table, which may be moveable using a planar motor. Reaction forces of the planar motor in the X, Y and Rz direction are transmitted to a balance mass. The balance mass is moveable over bearings that allow for movement of the balance mass in the X, Y and Rz direction with respect to a base plate, and does not allow movements, i.e., is stiffly coupled, in the Z, Rx and Ry directions with respect to the base plate. Reaction forces in all degrees of freedom of a fine positioning mechanism are exerted on the planar motor and transferred to the balance mass. The center of mass of the balance mass and the center of mass of an object table are at different positions in the Z-direction, the Z-direction being perpendicular to the X and Y directions. A consequence of the difference in center of mass is that when the planar motor exerts a horizontal force in the X and/or Y direction to the object table a torque around respectively the Y and/or X axes is generated in the balance mass. Because the balance mass is stiffly coupled to the base plate in the Rx and Ry directions through the bearings, this torque is transferred to the base plate. The torque may cause vibrations in the base plate, which deteriorates the functioning of the positioning system or a lithographic apparatus in which the positioning system may be used.